1. Field of the Invention
The present invention relates to an actuator for displacing a table through operation of a drive source to carry out mechanical work such as the conveying of a work-piece, and an actuator system which comprises a plurality of such actuators.
2. Description of the Related Art
There has heretofore been known an actuator having, as a drive source, an electric motor equipped with a sensor such as an encoder, a tachometer generator, or the like for making it possible to control a rotational speed, a drive torque, a stop position, or the like. The rotational motion from the electric motor is converted into linear motion by a drive force transmission shaft such as a ball screw, a trapezoidal screw, or the like, and the linear motion is transmitted to a table mechanism, which is displaced to convey a workpiece.
An electric motor which is incorporated in such an actuator is illustrated in longitudinal cross section in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the electric motor, generally denoted at 1, has a coupling 4 interconnecting an end of a ball screw 2 and an end of a motor shaft 3 coaxially with each other. The coupling 4 serves to absorb a misalignment between the axes of the motor shaft 3 and the ball screw 2, and prevent vibrations which are produced when rotary motion is converted into linear motion and also vibrations which are produced due to rotary motion containing a flexural component perpendicular to the axis as the ball screw 2 becomes longer, from being transmitted to the motor shaft 3.
One end of the ball screw 2 is rotatably supported by a first bearing 5 comprising balls 5a which are obliquely interposed between inner and outer races thereof. Since the balls 5a bear loads at a certain angle with respect to the axis of the ball screw 2 as indicated by the broken lines in FIG. 1, the balls 5a are capable of absorbing a load in a direction substantially perpendicular to the axis of the ball screw 2 and also a load in a direction substantially parallel to the axis of the ball screw 2.
The motor shaft 3 which is housed in a motor housing 6 has an end rotatably supported by a second bearing 7 and a spring washer 8, and an opposite end rotatably supported by a third bearing 9, which is of a double bearing configuration.
The first bearing 5 supports the ball screw 2 in both axial and radially inward directions. The second bearing 7 supports the motor shaft 3 in a radially inward direction for thereby absorbing vibrations and inertial forces that are generated in a radially outward direction by the rotational forces of the motor shaft 3. The third bearing 9 supports the motor shaft 3 in both axial and radially inward directions. Therefore, in the case where a photosensor P is mounted in the electric motor 1 for detecting the number of revolutions, the rotational speed, or the like of the electric motor 1, it is possible to position an encoder disk D fixedly mounted on the motor shaft 3 accurately within a clearance A in the photosensor P.
The electric motor 1 which tends to experience a relatively high temperature during operation suffers the problem of different thermal expansions due to different materials and shapes of the parts used. Typically, the motor housing 6 is made of an aluminum-base material for heat radiation, and the motor shaft 3 is made of an ironbase material. The difference between different thermal expansions of the materials of the motor housing 6 and the motor shaft 3 causes the motor housing 6 to be displaced axially, possibly concentrating stresses on the second bearing 7 which supports the motor shaft 3. Consequently, it is necessary to absorb the difference between these different thermal expansions in some way.
In the conventional electric motor 1, the spring washer 8 is interposed between balls 7a of the second bearing 7 and an inner wall surface of a bracket 6a of the motor housing 6. The difference between the different thermal expansions can be absorbed when the spring washer 8 is elastically deformed, pressing the balls 7a in a direction substantially parallel to the axis of the motor shaft 3.
Actuators for making rotary and linear motion, such as an electric actuator represented by an electric motor and a fluid pressure actuator represented by a fluid cylinder, are controlled by a motor driver and a solenoid-operated valve. These actuators, which include actuators for use in robots, are usually disposed independently of, not integrally with, a controller.
If the electric motor 1 is incorporated as an actuator in an apparatus (not shown), thus providing a drive source, it is necessary to reduce the size and weight of the actuator as much as possible in order to increase the versatility of the actuator.
The coupling 4 which interconnects the motor shaft 3 and the ball screw 2 may bring about resonance in the motor shaft 3 and the ball screw 2 when rotated. When the electric motor 1, which requires high dynamic characteristics, as with a servomotor, resonates, the positional control accuracy thereof is lowered, and its dynamic characteristics are impaired. If the coupling 4, which serves to prevent vibrations from being applied to the motor shaft 3, were dispensed with and the motor shaft 3 and the ball screw 2 were integrally coupled directly to each other in order to alleviate the above drawbacks, then unwanted vibrations would be transmitted to the motor shaft 3. As a result, it would be difficult to convey a workpiece continuously stably.